Using washers from a hardware store as 6ds to get endless games of table football at college, then finding that they also got accepted by old Underground ticket machines...
(For the yoof, 6d equalled 2.5p and could buy you a Mars bar about twice the size of the standard one today.)
And slamming train doors, immensely satisfying, even in the mobile-phone age..
My Dad could beat all this; he told me once as a kid (near Croydon) he and his best mate would pick up horse-dung (of which there was loads before card/vans took over) and threw it ON to all the people travelling on...